soil depth, a loose term for the depth to the boundary between the soil and its parent material; or, alternatively, the depth to which plant root growth is possible before reaching some impermeable barrier. Examples of the latter include the tight and/or poorly drained subsoils of many duplex soils (see soil), which occur in south-eastern Australia, South Africa, and the Piedmont region of the eastern US; cemented ironstone, or ‘coffee rock’, which forms at the base of some iron-rich podzolic soils that have been subject to leaching; or subsoils with poor structure due to exchangeable sodium, or excessively high concentrations of salt or acidity, or some other toxic factor which effectively prevents further root penetration.
